The Electronic Theatre Controls (ETC) Source Four (also known unofficially as Source 4 or S4) is an ellipsoidal reflector spotlight (ERS) used in stage lighting. First released in 1992, the Source Four was invented by David Cunningham and features an improved lamp and reflector compared to previous ERS designs, tool-free lamp adjustment, and a rotating, interchangeable shutter barrel. The Source Four is widely used by professional theaters across the globe. Glass reflector The Source Four uses a faceted borosilicate reflector behind the lamp. Nearly all stage lights have some form of reflector positioned behind the lamp to reflect otherwise wasted light out of the front of the instrument. The Source Four's reflector is dichroic, meaning that it reflects light of only certain wavelengths. The Source Four's reflector reflects back 95% of the visible light striking it, while allowing over 90% of the infrared radiation (heat) to pass out the back of the instrument. This produces a much cooler light which is less destructive to gobos or color gels at the front of the fixture and reduces localized heating of the lighting target. Lamp adjustment Lamp adjustment, or bench focus, is used to achieve an even field of light, and to remove hot-spots which can destroy color filters. On the Source Four, adjustment can be done without tools, and can be more accurate than older methods. The most common problem with lamp alignment is the lamp dropping too low in the reflector, causing a hot spot at the bottom of the beam, and a dark area at the top. This may be corrected by a realignment of the lamp. There are two adjustments that can be made to the cap. 1. The center screw controls the depth of the lamp into the reflector. Loosening this screw causes a spring to push the lamp further inside the reflector, creating a brighter hot spot in the beam. Tightening this screw will draw the lamp backwards, for a flatter, more even light field. 2. The wider nut that sits underneath the center screw moves the lamp's position horizontally and vertically. Loosening this nut will free the lamp assembly from the cap housing. The user can then push the entire adjustment screw and lamp along the plane to center the lamp in the reflector, evening the beam. Lamps HPL incandescent The proprietary HPL (High Performance Lamp) lamp uses a compact filament, which concentrates the most